Thermally augmented petroleum recovery, for example, hot water flooding, steam flooding and steam soaking operations, are widely practiced to recover oils with high in-situ viscosity and hence low mobility. Such oils are often found in rock formations having varying permeabilities and other heterogeneity that causes the hot fluid (water or steam) to enter one stratum or part of a stratum in preference to the others. In addition, gravity forces may cause the steam to rise to the top of a stratum and water to sink to the bottom of a stratum. The stratum which receives more flow becomes depleted of oil and no longer contributes to the economic benefit of the oil recovery process. Furthermore, because this stratum receives substantially all the heated injection or soak fluid, it is significantly hotter than other strata.
Numerous methods have been described for selectively plugging subterranean strata in oil recovery operations using delayed gel formation processes. U.S. Pat. No. 4,640,361 describes the application of thermally responsive aqueous silicate compositions, U.S. Pat. No. 4,785,883 describes the use of organic polysilicate esters in aqueous solutions, and U.S. Pat. No. 4,706,754 describes the application of aqueous solutions of a water-soluble polymer, a metal carboxylate crosslinking agent, and a delaying agent.
Several compositions particularly advantageous to steam soak and/or steam flood operations have also been described. U.S. Pat. No. 4,845,134 describes gellable aqueous compositions containing a monomer and one or more temperature sensitive crosslinkers for steam diversion. U.S. Pat. No. 4,907,656 describes a method of application of aqueous gellable compositions for preventing steam channeling into a non-aquifer bottom water zone.
U.S. Pat. Nos. 4,871,021 and 4,909,324 describe block copolymers useful in oil recovery operations. These describe aqueous solutions of block copolymers that can be crosslinked in high brine concentration reservoirs to yield high strength gels. Like the other crosslinkable polymers, they are water soluble and tend to concentrate in the aqueous regions of the formation.
Crosslinkers taught by the prior art in, for example, U.S. Pat. No. 4,909,324 include multivalent metal cations. These crosslinkers can be made to be temperature activated by the method taught in U.S. Pat. No. 4,907,656. This patent discloses utilizing a temperature dependent reduction of chromium to generate the crosslinking Cr.sup.+3 ion in-situ. Unfortunately, the aqueous environment of the highly permeable zones which are the targets of these gels dilute both the crosslinkers and the aqueous solutions of polymers. Hydrocarbon bearing zones, on the other hand, do not interfere with the crosslinking process. It would be preferable to have an oil phase polymer solution because the oil bearing strata would dilute any polymer solution which does reach it and diminish any crosslinking. The oil soluble polymers would also be less subject to hydrolysis by the aqueous environment of the highly permeable strata which are the targets of the profile control additives. Additionally, multivalent metal cations indigenous in the brines and rock formations will not have as great an effect on either the crosslinkers or the oil base polymers. Crosslinking would therefore be more predictable, controllable, and selective to the highly permeable strata if oil soluble polymers were utilized.
The known profile control polymer solutions have viscosities which decrease with increasing temperatures. But a more drastic change in viscosity between common low permeability strata temperatures and highly permeable strata temperatures would be preferred. This drastic change in viscosity would tend to permit greater penetration of polymer solutions into the highly permeable strata. This change in viscosity would also prevent penetration of lower temperature strata, permitting subsequent oil recovery.
An oil soluble crosslinkable polymer solution useful in well treating fluids is taught in U.S. Pat. No. 4,595,513. This composition is useful in fracture fluids and for transportation of masses of granular solids into voids in subterranean formations. The object of these polymeric additives are to provide a nearly constant viscosity over a broad temperature range. This is a significant improvement for fluids designed to transport solids due to the importance of viscosity in the suspension of solids. Profile control additives of the present invention have viscosities which are both time and temperature dependent.
It is therefore an object of the present invention to provide a process to preferentially decrease permeability cf more permeable strata of subterranean formations. It is a further object to provide such a process in which a polymer gel which is oil soluble is utilized, and in which the polymer gel is crosslinked selectively in the more permeable strata.